158 
ZOOLOGY: H. JORDAN 
period the time occupied from the beginning of one set of 'swinging 
motions' to the beginning of the next; thus the posterior orientation, 
which is of longest duration, is the last phase of the orienting process. 
A characteristic reaction, then, of Epinephelus striatus is posterior or 
lateral orientation to a current. Since in a majority of cases the orien- 
tation is posterior, and since this is maintained longer than any other 
position, it is a fair inference that this is the significant reaction. 
Various regions of the body were explored with a localized current 
(1/28 liter per second) directed through a long glass tube, the experi- 
menter being invisible to the fish. The following areas were found to 
be sensitive, but the response varied in promptness. Stimulation of 
the lips (if prolonged it causes a relatively violent reaction) brought 
forth a response in 7 seconds; the caudal fin in 16 seconds; the dorsal 
fin in 22, the cheek and operculum in 25, and the side of the body in 30 
seconds. From these observations it is clear that the Hp region is much 
the most sensitive part of the surface of the body, and the possibility 
is at once suggested that the posterior orientation of the fish is a reac- 
tion which serves to protect this region from the action of the current. 
This seems the more probable from the fact that prolonged stimulation 
of this region by the current employed in the experiment always caused 
a rather extraordinary and violent reaction. 
Several end organs have, at one time or another, been regarded as 
the receptors in the case of rheo tropic responses: lateral-line organs^ 
(Schulze, 70); the organs concerned in response to pressure^ (Ver- 
worn, '97, p. 445ff., barotaxis), and the ear^ (Tullberg, '03). 
Two other views have been advocated, first, that it is the tactile cor- 
puscles which are stimulated by currents^-^ (Parker, '03a, '03b) ; second- 
ly, that it is chiefly the eyes which are stimulated, this being due to the 
transportation of the fish through the water. According to this view^ 
(Lyon, '04), rheotropic reactions are chiefly optic reflexes, which serve 
to compensate the apparent motion of the visual field. This is an in- 
direct effect, a direct stimulation being produced only when the fish 
is in contact with some part of the solid environment. This, in the case 
of bhnd fishes, Lyon thinks acquaints the animal with its transporta- 
tion and a compensatory swimming results. 
In studying the question of rheotropic end organs, observations — to 
be described elsewhere — were made which confirm the idea that sense 
cells of pressure and equihbration are unaffected by water currents. 
The lateral-line organs and the eyes, which can be rendered function- 
less by appropriate operations, are also unaffected. The skin was 
next removed from certain body areas and the underlying tissue was 
